1. Field of the Invention
The invention relates to a process for reducing the iron content of CaO-rich slags formed during the desulphurization of crude iron. This invention also relates to a process for decreasing encrustations on the walls of ladles used in hot metal desulphurization.
2. Discussion of Prior Art
For many decades, crude iron has been desulphurized outside the blade furnace. In the earlier years, this was effected, in the main, by adding soda during the operation in which the crude iron is tapped from the blast furnace and run off into the transport ladles. In more recent years, crude iron desulphurization by the immersion-lance process has increasingly gained acceptance. In this process, the desulphurizing agents, e.g. calcium carbide or lime, are blown in with the aid of a lance which is plunged into the bath of crude iron. This blowing-in can be carried out in the ladle, in which the crude iron is transported from the blast furnace to the steelmaking unit.
The total costs of the so-called "external" crude iron desulphurization treatment, that is to say, the treatment performed outside the melting unit, are increased to a considerable extent by the losses of iron during the desulphurization treatment. Large quantities of finely-dispersed iron droplets are formed in the desulphurization slag, which apparently fail to coagulate to form larger drops, and which are accordingly unable to flow back into the bath of crude iron, but remain included within the slag. As a result of this, the crude iron desulphurization process is rendered very much less economic.
It has already been proposed to recover the iron from the desulphurization slag (German Patent Specification No. 750,334). However, this procedure for recovering iron from the desulphurization slag represents only a makeshift remedy. The iron is initially withdrawn from being directly further processed, and must be remelted after the recovery process.
When soda-free desulphurizing agents are used, the high iron contents in desulphurizing slags which are rich in CaO also lead to situations in which, when comparatively long delay times occur between the treatment of the melt and the emptying of the ladle, the slag, which is present in an essentially solid condition, forms comparatively large lumps as a result of the cooling and solidification of the iron which it contains. These lumps cake onto the wall of the ladle, and can be removed from the ladle only with difficulty. Especially in the case of torpedo ladles, this caking-on effect prevents the slag from flowing out with the crude iron, and an undesirable accumulation of slag occurs in the ladle, which reduces the usable ladle capacity. In order to prevent this effect, processes for removing slag deposits, occurring in the transport ladles during the desulphurization of crude iron, have been disclosed (German Patent Specification No. 2,247,475), but these processes have not proved successful.
It is an object of this invention to provide a process which keeps the iron content of the CaO-rich slags formed during the desulphurization of crude iron as low as possible, i.e., which inhibits migration of the iron into the slag. At the same time, it is an object of the invention to prevent the formation of slag deposits in the desulphurizing vessel.